Preparation And Sodium Storage Properties Of Heteroatom Doped Carbon Derived From Onion Waste

Carbonaceous materials have come to the stage as anode material for sodium ion batteries(SIBs),due to the low cost,high intrinsic conductivity,environmental benignity,and good structural stability.Furthermore,carbonaceous material with favorable structure can significantly alleviate the structural collapse,and thus promote the reaction kinetics and cycling stability.Biomass-derived carbons are attractive and cost-effective anode materials for SIBs,however,they generally suffer from the low capacity and coulombic efficiency.Foreign atomic doping has been proved to be an effective way to adjust the chemical properties and improve the electrochemical properties of carbon materials.Those dopants can enhance electrical conductivity,increase the active sites,expand interlayer spacing,and hence boost the intercalation/extraction of sodium ions during the electrochemical process.(1)The preparation of heteroatom nitrogen doped carbon(NC)and its properties in sodium storage.N(electronegativity:3.04,atomic radius:0.074 nm),relative to C(electronegativity:2.55,atomic radius:0.077 nm)has a larger electronegativity and a smaller atomic radius which can create an effective active site C+,but has an atomic radius close to C.So,it can be easily doped into the carbon SP2 lattice,forming a rich N-functional group,which is conducive to the charge transfer in the process of sodium ion insertion/removal.In this work,N-doped carbon spheres(NCs)were successfully prepared from onion waste and urea(CH4N2O)by hydrothermal and annealing process,with urea as cheap and additional N source.The thus-derived NC was used as anode material for SIBs,the optimal sample(NC-800)can harvest a reversible capacity of 152 mAh g-1 at a current density of 0.05 A g-1 after 200 cycles.It is impressive that the NC-800 can still provide 83 mAh g-1 capacity after 2000 cycles at 1 A g-1,and the capacity retention rate is 69%.The high capacity and excellent cycling of NC-800/NC-2 can be attributed to the following reasons:1)N doping improves the conductivity,increases the d002 layer spacing,increases the defects in carbon,shortens the ion diffusion channel,enhances the diffusion of sodium ions;2)structural stability,reduces the structural deformation during the circulation.This work puts forward a feasible method to convert low value waste into high value materials for electrochemical energy storage.This strategy can be extended to other kinds of sustainable biomass resources and applied to ordinary energy storage devices.(2)Preparation of nitrogen and sulfur co-doped carbon(NSC)and its sodium storage properties.We know that the electronegativity of N is higher than that of C,but the atomic radius is similar.The electronegativity of S(electronegativity:2.58,atomic radius:0.118 nm)is very close to that of C(electronegativity:2.55,atomic radius:0.077 nm).The influence of N and S co-doping on the structure and electrochemical properties of carbon is different from that of N-doped carbon alone.In this work,on the basis of the systematic study of mono nitrogen doped carbon,the N/S binary carbon material(NSC)was prepared by hydrothermal and annealing processes with onion as raw material,urea as nitrogen source and benzyl disulfide(BDS)as sulfur source.When NSC-800 is used as negative electrode material in SIBs,the reversible capacity of 99.7 mAh g-1 can be maintained after more than 200 cycles at a constant current density of 0.05 A g-1.The capacity of NSC-800 is 101,86.3,79.3,56.6,42.5 and 24.3 mAh g-1 at 0.05,0.1,0.2,0.5,1,2.5 C,respectively.When the current of 0.05 C is applied again,the capacity of NSC-800 is increased to 96 mAh g-1,keeping 95%of the initial capacity,showing good rate performance.This work provides a way of co-doping carbon with nitrogen and sulfur.It is found that the content of N and S doped in carbon cannot be increased at the same time.In the future work,we need to further study the role of N and S in electrochemical sodium storage,and discuss how to balance the content of N and S in carbon,so,as to obtain the NSC with excellent performance as the anode material of sodium ion battery.(3)The preparation of nitrogen and phosphorus co-doped carbon(NPC)and the preliminary exploration of its sodium storage properties.P(electronegativity:2.19,atomic radius:0.118 nm)has a lower electronegativity and a larger atomic radius than C(electronegativity:2.55,atomic radius:0.077 nm).p-doping can form the active site P+,and the insertion of P with a larger atomic radius into the carbon lattice will distort the SP2 plane of carbon,and further expand the carbon layer spacing.The structure of N and P-doped carbon is different from that of N-doped carbon,and the effect on electrochemical performance is different from that of N-doped carbon alone.In this work,on the basis of the above-mentioned one-way nitrogen doped carbon and two-way nitrogen/sulfur doped carbon,using onion residue as raw material,urea as nitrogen source,and tetraphenylphosphonium bromide(C24H20P(Br))as phosphorus source,the nitrogen/phosphorus co-doped carbon material(NPC)was prepared by a simple two-step method,and the electrochemical performance of the negative electrode material used in sodium ion battery was preliminary studied exploration.The sodium ion battery assembled with the best sample(NPC-800)can still provide 100 mAh g-1 reversible capacity after 200 cycles at the current density of 0.05 A g-1,which is almost similar to the performance of nitrogen and sulfur co-doped carbon(NSC-800),indicating that it has good cycling performance.This work provides a way of co-doping carbon with nitrogen and phosphorus,and preliminarily explores its sodium storage performance.However,the effects of different doping methods of nitrogen and phosphorus on the structure of carbon,the content of nitrogen and phosphorus,and the properties of sodium storage need further study.